It’s the stuff that dreams, and The Antiques Roadshow, are made of. A pensioner, as The Times reported yesterday, discovers that the two little pictures she hung in her spare room are a pair of lost portrait panels from an altarpiece by the 15th century master known as Fra Angelico. Sadly Jean Preston, the thrifty former academic who bought these glimmery little images as a present for her father in the early 1960s, died earlier this year, though not before she had discovered the great secret that her modest red-brick Oxford terrace house harboured.

But next spring when the pictures — bought for less than £200 the pair — come under the hammer, her heirs will have won the art world equivalent of the jackpot.

“It never ceases to amaze me how these things come to light,” gasped a delighted curator of early Italian paintings at the National Gallery.

Ms Preston wasn’t alone in discovering she had been unwittingly housing a major work of art. There has also been a lot of buzz around a new film documentary, Who the F*** Is Jackson Pollock?, which tells the story of a retired long-haul truck driver who found a wild, splotchy painting in a local thrift store and, after bartering with the shopkeeper, bought it for a bargain $5 (£2.60) — and then stuffed it in her garage because it wouldn’t fit through her door.

The film, taking its title from her plain-spoken response to the expert who suggested that it might be by the great Abstract Expressionist, follows her 15-year struggle to have the painting authenticated. This tale of a trailer-park denizen adrift in a snooty art world is certainly lent added drama in light of the recent sale of a classic Pollock drip painting for a record-breaking $140 million.

When it comes to the Queen, this type of story might be a rather less unexpected. Our monarch, after all, is the custodian of one of the world’s finest art collections. But still, the announcement a few days ago that a grimy old canvas dragged up for an overhaul from the Hampton Court storerooms has turned out to be a Caravaggio (rather than a copy of a lost painting by this master) must count as a thrilling plot twist.

This work, acquired for Charles I and (having been flogged off by Cromwell) recovered after the Restoration, has pretty much languished unloved ever since.

“It’s the most important discovery in the collection in the past decade,” declares an understandably excited Surveyor of the Queen’s Pictures, Desmond Shawe-Taylor.

But can we be sure that the latest attribution is right? Who are the people who can authenticate a painting? They certainly occupy a powerful position. The authenticity of an artwork lies at the core of its intrinsic and — more importantly for many — its monetary value.

Most masters have at least a few disputed authorships in their oeuvre. On the say of one person the worth of a painting can rocket or plummet overnight.

No wonder canvases have been the fields of such ferocious battles. Acrimonious legal wars have famously been fought over Renoirs and Rothkos, Picassos and Giacomettis, Calders and Braques. No wonder the National Gallery of Victoria in Melbourne was a bit miffed when, lending its precious Van Gogh portrait to an Edinburgh show last summer, it found it called into question. The picture extended its European stay, making a detour to Amsterdam so that Van Gogh museum experts could give the unknown man a full medical.

But processes of attribution are complex. There are three principal tools. First, experts look at the provenance: the historical documentation that, in a perfect world, will date from the time the painting left the studio to the present. In the case of the Queen’s Caravaggio, The Calling of Saints Peter and Andrew, this is fairly clear, but scholars can read the patchwork stamps and sale-room stickers on the back of a canvas like a story.

The second is visual inspection by connoisseurs, who combine acute sensitivity with historical awareness and empirical experience. These are the people who, once conservators had removed an alligator skin of bituminous varnish from the Caravaggio surface, spotted the incisions that this master typically made on his canvases to plot out positions, observed his characteristic attentiveness to ears, noted the pentimenti (the changes made by the artist during the course of the painting), studied the brushstrokes (more impetuous than a careful copyist might have risked) and the drama of a bravura composition.